The present invention relates to an optical transmission system for communication between two stations through an optical transmission means, in particular, relates to such a system for a communication between an optical subscriber unit in a center office (OSU) and a plurality of optical network stations (ONU) in customer premises coupled with said OSU through an optical means.
An OSU is a center station, and an ONU is a user station.
Conventionally, a passive double star (PDS) system has been known for time division multiplexing (TDM) communication through an optical means between a center station and a plurality of user stations.
FIG. 16 shows such a PDS system, in which an optical star coupler 1 which is a passive element is located at a joint portion, and a center station 2 is coupled with a plurality of user stations 3.sub.1 through 3.sub.n through said coupler 1. In said PDS system, a time compression multiplexing (TCM) is used for time division multiplex of an upward signal (U.sub.1, U.sub.2, U.sub.n) from a user station to a center station, and a downward signal (D.sub.1, D.sub.2, D.sub.n) from a center station to a user station. Simultaneously, a time division multiple access system (TDMA) is used for multiplexing a plurality of upward (downward) signals from (to) a plurality of user stations.
In a TDMA system, the transmission time between a center station and each user station is measured at the beginning of a communication so that transmission timing and communication capacity are assigned to each user station with no overlap of signals of the user stations. Each user station sends an optical signal at assigned timing, the optical signal is multiplexed passively in the optical star coupler 1, and the optical signals of each user stations locate on a time axis at a receive point of the center station 2. Similarly, a downward signal from the center station 2 to each user stations is splitted in the star coupler 1 so that each splitted signal is sent to each user station, which takes only a time slot addressed to the own station in the time division multiplexed signal.
The conventional PDS system has the disadvantage that a downward signal from a center station to user stations is attenuated in a star coupler depending upon the number of user stations, since a star coupler is a passive optical coupler in which a splitted signal is subject to attenuation relating to number of splitted channels. Similar attenuation would occur in an upward signal in a star coupler. Therefore, the requirements become severe for optical devices for transmission power, receive responsivity, et al installed in a center station and a user station, when the number of split in a star coupler is large.
Further, in a conventional PDS system, a frame signal in a downward signal must have an address for identification of each slot in a user station. The presence of an address makes a slot or a frame longer.
Further, since a downward signal is broadcast to all the user stations, a privacy facility or encription is essential. Also, an electronic circuit for processing said address and/or said privacy must be complicated.